The invention relates to elastic films comprising at least one and preferably two polyolefin thermoplastic components.
Ethylenexe2x80x94propylene copolymers and blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art. However, the traditional Ziegler-Natta catalysts used to make the ethylene propylene elastomer have limitations. Thus polymers which are simultaneously uniform in compositional distribution, have substantially stereospecific propylene residues and have less than 35 wt. % ethylene are not available with these catalysts. These limitations in the synthesis have lead to the absence of elastic films from blends of ethylene propylene copolymers and isotactic polypropylene.
U.S. Pat. No. 3,882,197 describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers.
U.S. Pat. No. 3,888,949 suggests the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term xe2x80x9cblockxe2x80x9d in the description of the polymer where the copolymer is described as having xe2x80x9csequences of different alpha-olefin content.xe2x80x9d Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
U.S. Pat. Nos. 4,461,872, improved on the properties of the blends described in 3,888,949 by using another heterogeneous catalyst system which is expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
Two successive publications in the journal of Macromolecules, 1989, V22, pages 3851-3866, described blends of isotactic polypropylene and partially atactic polypropylene which purportedly have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous and not crystalllizable. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described in U.S. Pat. No. 5,594,080, in the article in the Journal American Chemical Society (1995), 117, p. 11586; in the article in the Journal American Chemical Society (1997), 119, p. 3635; in the journal article in the Journal of the American Chemical Society (1991), 113, pp. 8569-8570, and in the journal article in the Journal Macromolecules (1995) 28, pp. 3771-3778. These articles describe the copolymer of the present composition but do not describe the compositions obtained in blends with a more crystalline polymer such as isotactic polypropylene, nor its resultant desirable physical properties.
U.S. Pat. Nos. 3,853,969 and 3,378,606, suggest the formation of in situ blends of isotactic polypropylene and xe2x80x9cstereo blockxe2x80x9d copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the xe2x80x9cstereo blockxe2x80x9d character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend of these patents.
Moreover, all of these compositions either do not meet all of the desired properties for various applications.
Similar results are purportedly achieved in U.S. Pat. No. 3,262,992 wherein the authors suggest that the addition of a stereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. These copolymers were synthesized by changing the monomer concentrations in the reactor with time. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60).
The presence of stereoblock character in these polymers is shown by the high melting point of these polymers and the poor solubility in hydrocarbons at ambient temperature.
Notwithstanding these descriptions of the polymer blends containing isotactic propylene segments it is apparent that useful articles such as elastic films have not been constructed from any of these materials. The utility of elastic films is that they (a) are soft to the touch, (b) can recover from temporary tensile deformation to essentially their original dimensions, this latter property may be of advantage in disposable garments to aid in retaining their shape. In addition, there is a need for elastic films which are easily processible in conventional thermoplastic plastics film equipment using conditions similar to that used for conventional thermoplastic films. Further, any or all of the conventional processes used for film fabrication should be usable to fabricate the elastic film blend. These include but are not limited to the following: compression molding, blown film extrusion and cast film extrusion. It is also further desireable to have elastic films composed essentially completely of polyolefins such that they are thermally stable, heat resistant, light resistant and generally suitable for thermoplastic applications.
There is a need therefore for elastic films composed generally completely of polyolefins but having simultaneously a crystalline stereospecific polypropylene component to obtain good tensile strength as well as a crystallizable ethylene-propylene copolymer to provide good elastic recoverability, resistance to elastic flow at a load sustained for specified period, as well as a glass transition temperature below that of polypropylene.
Embodiments of our invention include forming elastic films from predominantly crystallizable, semicrystalline polyolefin polymers. Further, embodiments include improving the aforementioned properties of films by blending a generally minor amount of a crystalline polyolefin where the type of crystallinity of the two components are similar, as for instance both will be substantially isotactic or syndiotactic, but the amount of crystallinity differs. Isotactic and syndiotactic arrangement of monomers in a polymer are defined in xe2x80x9cPrinciples of Polymerizationxe2x80x9d by G. Odian (3rd Ed), 1991, p. 607 (John Wiley) which is incorporated herein by reference. Substantially pertains to an arrangement of monomer units where greater than 50% of adjacent monomer units have the defined tacticity. Other embodiments of our invention are directed to polyolefins and polyolefin blends where the crystallizable and crystalline components have a stereoregular polypropylene component, especially preferred is isotactic polypropylene. A crystalline polymer is one with a heat of fusion, a measured by Differential Scanning Calorimetry (DSC) to be greater than 50 J/g. A crystallizable polymer is one with a heat of fusion, as measure by DSC, to be less than 50 J/g. In the semicrystalline, crystallizable polymer this is achieved with a crystallizable copolymer of propylene and a C2, C3-C20 alpha-olefin, preferably propylene and at least one other alpha-olefin having less than 6 carbon atoms, and more preferably propylene and ethylene. Improvements in the properties of the semicrystalline, crystallizable copolymer can be obtained by blending it with the crystalline stereoregular polypropylene component, particularly isotactic polypropylene. This crystallizable copolymer is less crystalline than the crystalline isotactic polypropylene. In the crystallizable copolymer the propylene is polymerized substantially stereospecifically. Preferably, the crystallizable copolymer is an ethylene propylene crystallizable copolymer, e.g., ethylene propylene elastomer that is thermoplastic. The crystallizable copolymer has a substantially uniform composition distribution, preferably as a result of polymerization with a metallocene catalyst. Composition distribution is a property of these crystallizable copolymers indicating a statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later.
We have found that a crystallizable, semicrystalline propylene alpha olefin crystallizable copolymer, hereinafter referred to as the xe2x80x9cfirst polymer componentxe2x80x9d (FPC) can be used to make elastic films. The properties of the film can be improved by blending an amount of a crystalline propylene polymer, hereinafter referred to as the xe2x80x9csecond polymer componentxe2x80x9d, (SPC). These blends have the advantageous processing characteristics while still providing a composition having decreased flexural modulus and increased adjusted or normalized load capacity and low values of set and load decay. The decrease in set and load decay refer to the ability of the elastic film to withstand instantaneous and sustained loads, respectively, without substantial deformation. These improvements are most apparent as a function of the 200% tensile modulus (designated as load in the data below) of the blend. Historically, the examples of the prior art have been able to duplicate the improvements in the blend but only for compositions with a very low 200% tensile modulus.
It is possible to have a third polymeric component which is another crystallizable propylene alpha olefin copolymer indicated as FPC2 in the text below, which has crystallinity intermediate between the FPC and the SPC. The FPC2 also has a narrow composition distribution and is made with a metallocene catalyst. The addition of FPC2 leads to a finer morphology of dispersion of the FPC and improvements in some of the properties of the blend of FPC and SPC. In addition, these blends for elastic films may contain non-polymeric ingredients such as process oil, inorganic components such as particulate fillers such as carbon black, mica or calcium carbonate.
The term xe2x80x9ccrystalline,xe2x80x9d as used herein for SPC, characterizes those polymers which possess high degrees of inter- and intra-molecular order.
FPC2 describes those polymers or sequences which are substantially crystalline in the undeformed state (however, less crystalline than the SPC). Further crystallization may also occur in the presence of the crystalline polymer such as SPC.
The blends and the films made therefrom, contain a continuous phase of low crystallinity. For blends containing at least two polymeric components, an additional dispersed phase of greater crystallinity is also present. In this latter case the sizes of the individual domains of the dispersed phase are very small with the smallest length dimension for the dispersed phase being less than 5 xcexcm. This phase size of the dispersed phase is maintained during processing even without crosslinking. The dispersed phase consists of a crystalline mixture of SPC with some amount of FPC2 (when present in the blend) and FPC, due to thermodynamic mixing of polymers. The continuous phase consists of the balance of the polymers not included in the dispersed phase. Blends directed to low flexural modulus may have in addition, a heterogeneous phase morphology with continuous phases of lower and greater crystallinity.
Commonly available reactor copolymer consisting of a single phase blend of isotactic polypropylene and copolymers of propylene and ethylene are not included within the scope of the present invention since they are a single phase with no prominent dispersed or continuos phases. Impact copolymer, thermoplastic olefins and thermoplastic elastomers have heterophase morphology made by a combination of a SPC and a FPC of the present invention. However, the more crystalline polymer is the continues phase in these blends and they are excluded from the present invention. The components of the blend in both cases are also compatible to the extent that no preformed or insitu formed compatibilizer needs to be added to attain and retain this fine morphology. Furthermore, embodiments of this invention describe improving the mechanical deformation recoverability of the aforementioned blends by annealing the blends and/or mechanically orienting the films formed from these blends.
The films made from these blends are made either by casting, compression molding or blowing films or by any of the other procedures known in the art. Typically, these films are between 0.1 to 100xc3x9710xe2x88x923 inch in thickness.
A preferred composition for the formation of the elastic films comprises 0%-95% by weight, preferably 2%-30% by weight and more preferably 2%-25% by weight of SPC and the balance of the composition being the FPC. The FPC comprising a crystallizable copolymer of the FPC of the invention, has isotactically crystallizable propylene sequences with a crystallinity less than 40 J/g and greater than 65% by weight propylene and preferably greater than 80% by weight propylene.
According to another embodiment, a thermoplastic polymer blend for the preparation of the elastic film composition of the invention comprises a SPC and a FPC with added process oil. The SPC comprises isotactic polypropylene, a reactor copolymer or an impact copolymer as described above. The balance of the polymer blend composition may consist of a mixture of the process oil and the FPC and FPC2 if used.
Still further embodiments of our invention are directed to a process for preparing thermoplastic blends suitable for the preparation of elastic films is contemplated. The process comprises: (a) polymerizing propylene or a mixture of propylene and one or more monomers selected from C2 or C3-C20 alpha olefins in the presence of a polymerization catalyst wherein a substantially isotactic propylene polymer containing at least 90% by weight polymerized propylene is obtained; (b) polymerizing a mixture of ethylene and propylene in the presence of a chiral metallocene catalyst, wherein a crystallizable copolymer of ethylene and propylene is obtained comprising up to 35% by weight ethylene and preferably up to 20% by weight ethylene and containing isotactically crystallizable propylene sequences; and (c) blending the propylene polymer of step (a) with the crystallizable copolymer of step (b) to form a blend. Prochiral catalysts suitable for the preparation of crystalline and semi-crystalline polypropylene copolymers include those described in U.S. Pat. Nos. 5,145,819; 5,304,614; 5,243,001; 5,239,022; 5,329,033; 5,296,434; 5,276,208; 5,672,668; 5,304,614; and 5,374,752; and EP 549 900 and 576 970, all incorporated herein by reference. Additionally, metallocenes such as those described in U.S. Pat. No. 5,510, 502 (incorporated herein by reference) are suitable for use in this invention.
According to still a further embodiment, the invention is directed to a process for preparing of elastic films from these thermoplastic polymer blends. The process comprises: (a) generating the thermoplastic blend (as described immediately above), (b) forming the elastic film by casting, blowing or compression molding as described in the art, (c) annealing the resulting films for a period of time less than 20 days at a temperature not to exceed 170xc2x0 C. and (d) orienting the film either uniaxially or biaxially by extension to not greater than 700% of its original dimension. The annealing and/or the orientation may be conducted in a single operation or as distinctive sequential operations.
Another embodiment includes a film including a blend of polymers, the film has excellent resistance to set and excellent resistance to load decay, the blend of polymers being substantially noncrosslinked, comprising a first polymer component (FPC), the FPC has:
i) a composition distribution such that at least 75 weight percent of the polymer is isolated in two adjacent soluble fractions, each of these fractions has a composition difference of no greater than 20% (relative) of the average weight percent ethylene content of the whole first polymer component;
ii) a melting point, as determined by differential scanning calorimeter (DSC) less than 105xc2x0 C.;
iii) a heat of fusion less than 45 J/g;
iv) propylene and an alpha-olefin present in said FPC, wherein the alpha-olefin is present in the FPC from 4-35 weight percent, wherein the alpha-olefin is selected from the group consisting of ethylene and C4-C12 alpha-olefin, the propylene making up the balance of the FPC. The FPC is present in the blend in the range of from 5-100 weight percent.
Also included is a second polymer component (SPC), the SPC being crystalline polymer having:
i) a melting point above 115xc2x0 C.;
ii) a heat of fusion above 60 J/g;
iii) propylene present at least 90 weight percent, and an alpha-olefin present at less than 9 weight percent, wherein the total of the propylene and the alpha-olefin in the SPC adds to 100 weight percent;
the SPC being present in the blend in the range of from 0-95 weight percent;
wherein the film exhibits a resistance to set equal to or less than that described by the equation:
Set=7+[9/1000]xc3x97 Adjusted Load (L2);
and
xe2x80x83
wherein the film exhibits a load decay that is equal to or less than 20%.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.